1. Field
The disclosed embodiments relate to a heat exchanger intended to be positioned above a pylon that connects a jet engine to a wing structure of an aircraft, the said heat exchanger being capable, at least partially, of cooling a very hot air stream using a stream of cold air. More specifically, the disclosed embodiments relate to a hot air inlet pipe and a cooling air inlet pipe carrying the hot air stream and the cooling air stream through an exchanger unit. The disclosed embodiments also relate to a propulsion system comprising such a heat exchanger. The disclosed embodiments also relate to an aircraft comprising at least one propulsion system according to the disclosed embodiments.
2. Brief Description of Related Developments
In the field of aeronautics it is known practice to use hot air, bled from the jet engine compressors of an aircraft, in the air conditioning circuits of the said aircraft. Because the air conditioning circuits are intended for the flight deck and passenger cabins, it is necessary to cool this hot air before it can be injected into the said circuits.
To do this, use is made of a heat exchanger in which the hot air stream crosses a stream of cold air bled from the outlet side of a fan belonging to the jet engine. The cold air stream and the hot air stream flow inside a unit of the heat exchanger, generally of parallelepipedal shape, so that a heat exchange can take place.
In general, the hot air stream enters the heat exchanger unit via a front face and flows from front to rear therein, inside hollow plates. The partially cooled hot air stream re-emerges from the unit via a rear face to be directed to the air conditioning circuit. The cold air stream, for its part, enters the heat exchanger unit via a lower face and flows from the bottom up, between the hollow plates, and is then discharged from the unit by openings formed on an upper face, to be removed from the engine pylon.
The cold air stream and the hot air stream are therefore perpendicular to one another in the heat exchanger unit, which means that as they leave the heat exchanger, the exchange of heat has been somewhat inefficient.
The hot air that needs to be cooled is bled from the jet engine situated under the pylon. It is therefore necessary, in order to carry this hot air stream into the heat exchanger, to cause it to pass through the structure of the pylon. In order not to weaken the structure of the pylon it is absolutely essential to maintain its integrity as far as possible, in particular by avoiding having too many pipes passing through it.
Usually, the partially heated cold air is discharged from the jet engine directly. It therefore plays no further part in the thermodynamic cycle of the engine, this leading to a loss of engine performance. This partially heated cold air, as it flows over the pylon that attaches the jet engine to the wing structure of the aircraft, disturbs the aerodynamic flow of the aircraft. What is more, it is necessary to use a material capable of withstanding the high temperatures or to cover the engine cowls with a heatproof coating to prevent the heated cold air from damaging the said cowls. This tends to increase the mass of a propulsion system comprising a heat exchanger. It is also known practice for the stream of cooling air that has flowed through the heat exchanger to be reinjected into the jet stream providing the thrust. To do that, the cooling air discharge pipe has to pass through the pylon to reach the jet engine, and this weakens the structure of the pylon.